Adaptive equalizer for variable length sound tubes utilizing an acoustical time of flight measurement

ABSTRACT

In a device having a sound tube and a receiver, a test signal is generated to apply to the receiver. An outgoing acoustic wave is created at the receiver from the test signal. A reflected acoustic wave is received at the receiver, wherein the reflected acoustic wave is delayed in time from the outgoing acoustic wave. A difference in time is measured between the outgoing acoustic wave and the reflected acoustic wave. The difference in time is used to estimate a length of the sound tube.

REFERENCE TO RELATED APPLICATIONS

The present application is related to the following U.S. applicationscommonly owned together with this application by Motorola, Inc.:

Ser. No. 10/33/284 T. B. filed Dec. 27, 2002, titled “Adaptive Equalizerfor Variable Length Sound Tubes Utilizing an Acoustic Pressure ResponseMeasurement” by Willis; and Ser. No. 10/33/281 T. B. filed Dec. 27,2002, titled “Adaptive Equalizer for Variable Length Sound TubesUtilizing an Electrical Impedance Measurement” by Willis.

FIELD OF THE INVENTION

The present invention relates generally to an adaptive equalizer forvariable length sound tubes utilizing an acoustical time of flightmeasurement.

BACKGROUND OF THE INVENTION

Some users of earpiece accessories have a strong preference for certaintypes of sound delivery systems over others (e.g., an accessory sounddelivery tube that terminates in the ear canal versus an intraconcha(inside the bowl of the ear) device). The equalization used to controlacoustical standing waves in the earpiece tubing has been shown to havea key role in lowering the threshold of feedback in the earpiece as wellas improving the quality of the audio presented to the user. Thestanding wave frequencies are a function of the combined length of thesound tube and accessory tube; and therefore, the equalization must betuned to the particular delivery system used. For example, an accessorythat terminates near the entrance to the ear canal results in a totaltube length different than one that is inserted into the ear canal. Thisrequires a realignment of the equalization in order to maintain feedbacksuppression and optimum sound quality.

Thus, there exists a need for being able to recognize the length of theaccessory sound tube that is connected to the earpiece in order toselect the proper settings for the equalizer.

BRIEF DESCRIPTION OF THE FIGURES

A preferred embodiment of the invention is now described, by way ofexample only, with reference to the accompanying figures in which:

FIG. 1 illustrates a simple block diagram of an earpiece for theacoustic pressure response technique in accordance with the presentinvention;

FIG. 2 illustrates a graph of sound pressure level produced by areceiver for various tubing lengths in accordance with the presentinvention;

FIG. 3 illustrates a simple block diagram of an earpiece for theelectrical impedance and time of flight measurement techniques inaccordance with the present invention; and

FIG. 4 illustrates a graph of the electrical impedance of a receiver forvarious tubing lengths in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements are exaggeratedrelative to each other. Further, where considered appropriate, referencenumerals have been repeated among the figures to indicate identicalelements. For clarity, the present invention defines the sound tube asthe section of tubing inside the earpiece, and the accessory as thesection of interchangeable tubing outside the earpiece.

The present invention proposes a solution for selecting the propersetting for an equalizer (realigning the equalization) based on thelength of tubing coupled to a device (e.g., an earpiece). The term“tubing” will refer to the total length of tubing (sound and/oraccessory tubes) coupled to the device. Typically, the sound tube is atleast partially internal to the device. Thus, the sound tube may have anadjustable length or may have an interchangeable accessory attachedthereto. As noted above, realigning the equalization based on the lengthof tubing maintains feedback suppression and optimizes sound quality.The present invention focuses on three techniques for estimating thetotal length of tubing coupled to the device: an acoustic pressureresponse technique, an electrical impedance response technique, and atime of flight measurement technique. Once the total length of tubing isestimated, the present invention is able to select the proper settingsfor the equalizer.

Let us first discuss the acoustic pressure response technique. FIG. 1illustrates an example of a simple block diagram of an earpiece 100. Asshown, when a receive audio signal 102 enters the earpiece 100, it isprocessed by a digital signal processor (“DSP”) 104; the DSP 104comprises an equalizer 106 for equalization of the receive audio signal102 to compensate for the acoustical response of a receiver 108, soundand/or accessory tubes 110. The receive audio signal 102 is convertedinto an analog electrical signal 112 by a digital-to-analog (“D/A”)converter 114. The analog electrical signal 112 is fed into an amplifier116, and the output of the amplifier 116 is coupled to the receiver 108.The receiver 108 converts the analog electrical signal 112 to anacoustical output 118. The sound and/or accessory tubes 110 are coupledto the receiver 108 to deliver the acoustical output 118 to the user'sear canal 120. Typical responses of the acoustic pressure delivered tothe user's ear canal 120 for varying lengths of tubing (i.e., thecombination of both the sound tube and the accessory) 110 areillustrated in FIG. 2.

The send audio path consists of a microphone 122, which picks up theuser's voice 124. The output of the microphone 122 is fed into ananalog-to-digital (“A/D”) converter 126, which sends a digitized signal128 to the DSP 104. The digitized signal 128 is passed to the sendoutput 130.

Acoustical coupling exists between the receive and send audio paths dueto acoustic leakage 132 resulting from sound leaking out of the user'sear 120 and finding its way into the microphone 122. The presentinvention takes advantage of the acoustic leakage 132 by using themicrophone 122 in the earpiece 100 to measure the acoustic pressureresponse of the sound and/or accessory tubes 110 when a test signal (notshown) is played through the receiver 108. The present inventionestimates the total length of the tubing (i.e., the sound tube and/orthe accessory) 10 by the frequency of at least one peak in the acousticpressure response and the sound speed in the tubing 110. The presentinvention assumes that the sound speed in the tubing 110 isnon-dispersive.

In order to estimate the total length of the tubing in accordance withthe present invention, a test signal is needed. The test signal maycomprise a sequential set of stepped tones. Generation of the test tonescan be simplified in the DSP 104 by using square waves close to thehighest frequency peak in the acoustic pressure response. Since thispeak is near the upper limit of the earpiece passband, the square waveharmonics will fall well outside the passband and be attenuated. Thealgorithm then steps through a number of frequencies surrounding thepeak. From this data, a curve fit of the acoustic pressure response overa limited frequency band surrounding the highest frequency peak can beconstructed. The maximum of the curve fit will be at a resonancefrequency of the sound tube. This frequency is then used to tune theequalizer response to the attached accessory.

Using the lengths of the sound and/or accessory tubes 110, an equalizeris selected or designed. For example, if a lookup table is used, thelength of the sound and/or accessory tubes 110 will identify whichaccessory has been attached to the earpiece 100. The appropriateequalizer for that accessory can be loaded into the DSP 104 to optimizeaudio quality and earpiece stability. Alternatively, the lengths of thesound and/or accessory tubes 110 may be used to design an equalizer;preferably, a model of the acoustic pressure response, in which thetubing length is an adjustable parameter, is used to define an inverseequalizer to compensate for the responses for the receiver 108 and soundand/or accessory tubes 110.

Let us now move the discussion to the electrical impedance responsesolution in accordance with the present invention. A similar method tothe acoustic pressure response technique as described above can be doneusing the electrical impedance of the receiver. The electrical impedanceresponse of the receiver is strongly influenced by the acousticalloading of the standing waves in the tubing. As illustrated in FIG. 3,measuring the electrical impedance response requires the addition of aresistor 300 in series with the receiver 108, and a double pole, doublethrow switch 302 to bypass the resistor 300 during normal operation ofthe earpiece 100 and to select the input to the A/D converter 126 inorder to measure the voltage across the receiver 108; alternatively,current could also be measured through the receiver.

With the switch 302 in test mode, such that the resistor 300 is notbeing bypassed and the A/D converter 126 is measuring the voltage acrossthe receiver 108, the voltage measured across the receiver 108 is inproportion to the electrical impedance response. An example of theelectrical impedance response of a balanced armature receiver isillustrated in FIG. 4. Although the inductive and capacitive elements inthe electro-acoustic model of the receiver prevents the minimums andmaximums in the electrical impedance response from coinciding with thestanding wave frequencies of the tubing, the minimums and maximums inthe electrical impedance response illustrated in FIG. 4 shift inaccordance with the standing wave frequencies in FIG. 2. Using thesquare wave technique described in the acoustic pressure responsemethod, shifts in the resonant frequencies of the tubing can bedetermined from the electrical impedance response of the receiver. FIG.4 illustrates that the electrical impedance response is much moresensitive to the standing waves than the acoustic pressure response.

The total length of the sound and/or accessory tubes 110 is estimated bythe shift in the frequency of at least one peak in the electricalimpedance response. This requires that the tubing length is known apriori for one corresponding peak frequency in the electrical impedanceresponse. Using the length of the sound tube and accessory together, anequalizer is selected or designed as described above.

The third technique, the time of flight measurement, estimates thelength of the sound and/or accessory tubing by measuring the timerequired for an acoustical pulse produced by the receiver 108 topropagate down the length of the tubing, reflect at the open end of theaccessory, and return to the receiver 108. The length of the tubing isone half the time of flight to the end of the tubing and back to thereceiver multiplied by the speed of sound in the tubing, where the speedof sound is constant over the length of the tubing. A more complicatedequation results if the sound speed varies along the length of thetubing.

The hardware configuration required for the time of flight measurementis the same as for the electrical impedance response, however, thereceiver 108 is used as both an acoustical source and receiving device.The resistor 300 is necessary in this case to prevent the returningpulse from being shorted out by the low output impedance of theamplifier 116. The acoustical pulse must be shorter than the timerequired for the pulse to travel down the length of the tubing, reflectat the open end of the accessory, and return to the receiver 108.

In operation, using the time of flight technique, the DSP 104 generatesa test pulse 302. The test pulse 302 exits the D/A converter 114 andgoes through the series resistor 300. At this point, the A/D converter126 detects the outgoing test pulse. The receiver 108, which is inparallel with the AID converter 126, receives the test pulse at the sametime as the AID converter 126, and generates an acoustic pulse 304 thattravels down the length of the tubing 110, reflects 308 at the open end306 of the accessory and travels back to the receiver 108. A voltagedevelops across the series resistor 300 produced by the receiver 108that now acts as a dynamic microphone. The A/D converter 126 detects thereflected pulse 308. The difference in time between the outgoing testpulse 302 and its reflection 308 is used to calculate the length of thetubing.

While the invention has been described in conjunction with specificembodiments thereof, additional advantages and modifications willreadily occur to those skilled in the art. The invention, in its broaderaspects, is therefore not limited to the specific details,representative apparatus, and illustrative examples shown and described.Various alterations, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. Thus, itshould be understood that the invention is not limited by the foregoingdescription, but embraces all such alterations, modifications andvariations in accordance with the spirit and scope of the appendedclaims.

I claim:
 1. In a device having a sound tube and a receiver, a methodcomprising the steps of: generating a test signal to apply to thereceiver; creating an outgoing acoustic wave at the receiver from thetest signal; receiving a reflected acoustic wave at the receiver therebynow acting as a dynamic microphone, and wherein the reflected acousticwave is delayed in time from the outgoing acoustic wave; measuring adifference in time between the outgoing acoustic wave and the reflectedacoustic wave; using the difference in time to estimate a length of thesound tube; and selecting an equalizer based on the length of the soundtube.
 2. The method of claim 1 wherein the outgoing acoustic wavepropagates down the sound tube and is reflected at an end of the soundtube.
 3. The method of claim 1 wherein the sound tube has an adjustablelength.
 4. The method of claim 1 wherein the sound tube comprises aninterchangeable accessory.
 5. The method of claim 1 wherein theequalizer is selected from a lookup table.
 6. The method of claim 1wherein the test signal comprises a sequential set of stepped tones. 7.The method of claim 1 wherein the receiver is an acoustical source and areceiving device now acting as a dynamic receiver.
 8. The method ofclaim 1 wherein the device is an earpiece.
 9. The method of claim 1wherein the sound tube is internal to the device.
 10. The method ofclaim 1 wherein the sound tube is external to the device.
 11. The methodof claim 1 wherein a first portion of the sound tube is internal to thedevice and a second portion of the sound tube is external to the device.12. In a device having a sound tube and a receiver, a method comprisingthe steps of: generating a test signal to apply to the receiver;creating an outgoing acoustic wave at the receiver from the test signal;receiving a reflected acoustic wave at the receiver thereby now actingas a dynamic microphone, and wherein the reflected acoustic wave isdelayed in time from the outgoing acoustic wave; measuring a differencein time between the outgoing acoustic wave and the reflected acousticwave; using the difference in time to estimate a length of the soundtube; and designing at least a portion of an equalizer based on thelength of the sound tube.